Survival of Prosthodontic Restorations Luted with Resin-Based versus Composite-Based Cements: Retrospective Cohort Study

The purpose of this study was to evaluate clinical performance, survival, and complications of indirect composite inlays, onlays, and overlays on posterior teeth. Digital records of 282 patients treated between 2014 and 2018 were accessed and analyzed retrospectively. The included patients received 469 composite restorations luted with seven different resin-based types of cement, i.e., Filtek Ultimate Flow, Enamel Plus, Relyx Ultimate, Harvard Premium Flow, Relyx Unicem, Filtek Bulk Fill Flowable, and Filtek Ultimate. The restorations had been clinically and radiographically evaluated annually. The mechanical and clinical complications, e.g., debonding, fracture, and secondary caries, were evaluated and recorded. The examined restorations exhibited a high survival rate (84.9%), and failure was found in only 71 cases. Fracture was the most common cause (n = 36), followed by prosthetic work release (n = 19) and secondary caries (n = 16). There was a statistically significant difference between failure and cement material (Sig. < 0.001); the composite-based cements (87.2%) had a high survival rate compared to the resin-based cement (72.7%). Similarly, the cements with high viscosity (90.2%) had significantly higher survival rates than the low-viscosity cements (78.9%). Moreover, onlays showed higher longevity compared to overlays (Sig. = 0.007), and patients aged under 55 years showed less complications (Sig. = 0.036). Indirect composite restoration was a successful solution to tooth structure loss. The material of the cementation is an important part of the success. Higher survival rate was found in our study when the fixation materials with high viscosity were used, thus suggesting using these materials with indirect restorations. Composite-based cements had significantly higher survival rate than resin-based cements.


Introduction
In clinical practice, hard dental tissue loss caused by decay, erosive or abrasive wear, trauma, or a combination often exceeds direct restoration limits. According to Pjetursson et al. 2008, the preparation for conventional crown luted with conventional cement leads to additional loss of dental tissues [1]. In a recent systematic review, Angeletaki et al. 2016 concluded that the indirect approach is recommended when the cusp is lost or the defect exceeds the occlusal third [2].
Indirect composite restorations offer better mechanical properties than direct composite restorations, such as higher wear resistance and lower polymerization shrinkage [3]. The indirect process of fabrication offers better access for creating natural occlusal and interproximal design. While the degree of conversion influences fracture resistance and of the restoration while the shrinkage is larger compared to the conventional composites [43]. One the advantages of using flowable composites is creating the so-called "super dentin" and acid-resistant zone, which can prevent secondary caries [44,45]. Flowable composites had been suggested as alternatives to dual-cured resin cements when the restoration is thinner than 2 mm. There is a huge variability within the group of flowable composites according to radiopacity, flowability, filler content, modulus of elasticity, and flexural strength ranging from 66 to 145 MPa [46][47][48][49][50].
Preheating composites makes the placement of restoration easier, the conversion of the monomer higher, and the optical proprieties maintained [51,52]. However, preheating of the composites allows better adaptability and it leads to an increased shrinkage. The volume of the preheated composite layer is higher compared to the resin-based cements or flowable composites and the marginal gap can be increased [42,[53][54][55]. Both the preheated composite and flowable composite can be used with the total-etch protocol. The self-etch protocol does not require removal of the smear layer, but it modifies the smear layer and incorporates it [56].
In light of the previous findings, this study was designed to study indirect composite restorations due to the lack of evidence-based recommendations for the most appropriate luting cement used with composite restorations. The overarching aim of this study was to describe the outcomes of composite restorations luted with different cement materials over five years of post-installation follow-up.

Design
This is a retrospective cohort study, for which the patients received indirect prosthetic restorations luted with different resin-based cements and they were checked annually. The study was designed, conducted, and reported according to the Strengthening the Reporting of Observational studies in Epidemiology (STROBE) guidelines for cohort studies [57] (Supplementary Materials Table S1).

Setting
The study protocol was reviewed and approved by the Ethics Committee of the Faculty of Medicine and Dentistry, Palacký University Olomouc Ref. No. 223/21. All the patients were recruited and treated at an academic specialty facility, the Department of Prosthodontics, Palacky University Hospital in Olomouc, the Czech Republic, between January 2014 and October 2018. Each patient was checked annually, and all the prosthetic restorations were fabricated by one dental laboratory affiliated with the university hospital. The clinicians who delivered the restorations followed the same protocol.
Preparation of the teeth was performed under local anesthesia according to conventional principles for adhesive onlay preparation. At least one cusp was covered and the convergence angle was around 10 • , with the limitation of free-hand preparation. The margins were prepared as a butt joint. After the preparation polishing, the immediate dentin sealing was performed under rubber dam and a eugenol-free provisional filling was placed.
The composite onlays were fixed using the following protocol; after the initial try-in, the restoration was sandblasted on the inner surface with aluminum dioxide 25 µM and cleaned with an ultrasonic cleaner for 2 min and air dried, and a silane agent (Ultradent) was applied for one minute, then the rubber dam was placed [58,59]. The inner surface of the restoration was treated with the adhesive system without polymerization. The tooth surface was prepared in accordance with manufacture recommendation for luting material. Composite that was used to block undercuts or as a coronal seal was sandblasted with aluminum dioxide 25 µM, then rinsed for 40 s and air dried. The tooth surface was prepared according to cement type. The excesses were removed using a scalpel, micro brush, super floss, and were eventually polymerized for 20 s six times from different aspects of the tooth. Occlusion was checked and the fit was checked with radioisotope thermoelectric generator (RTG).

Sample
A total of 282 patients received 469 composite inlays, onlays, and overlays on either posterior vital or nonvital teeth following the manufacturers' instructions and the guidelines for fixation in adhesive dentistry of Mante et al. 2013 [21]. The participants were included in this study according to the following eligibility criteria (Figure 1). prepared according to cement type. The excesses were removed using a scalpel, micro brush, super floss, and were eventually polymerized for 20 s six times from different aspects of the tooth. Occlusion was checked and the fit was checked with radioisotope thermoelectric generator (RTG).

Sample
A total of 282 patients received 469 composite inlays, onlays, and overlays on either posterior vital or nonvital teeth following the manufacturers' instructions and the guidelines for fixation in adhesive dentistry of Mante et al. 2013 [21]. The participants were included in this study according to the following eligibility criteria (Figure 1). The inclusion criteria were (a) vital or nonvital premolars or molars (posterior teeth), (b) the prosthetic work should be either inlay, onlay, or overlay, (c) the recipient tooth should have at least one adjacent and one antagonist tooth, and (d) rubber dam use to ensure isolation.
The exclusion criteria were (a) irregular dental attendants or delegated patients with no follow-up data, that is the patients who started the study but they did not visit prosthodontists for follow-up; therefore, they were depicted as dropped out from the study (n = 76); (b) severe systemic diseases or severe salivary gland dysfunction (n = 3); (c) parafunctional habits, e.g., grinding and thumb-sucking (n = 29); (d) low level of oral hygiene The inclusion criteria were (a) vital or nonvital premolars or molars (posterior teeth), (b) the prosthetic work should be either inlay, onlay, or overlay, (c) the recipient tooth should have at least one adjacent and one antagonist tooth, and (d) rubber dam use to ensure isolation.
The exclusion criteria were (a) irregular dental attendants or delegated patients with no follow-up data, that is the patients who started the study but they did not visit prosthodontists for follow-up; therefore, they were depicted as dropped out from the study (n = 76); (b) severe systemic diseases or severe salivary gland dysfunction (n = 3); (c) parafunctional habits, e.g., grinding and thumb-sucking (n = 29); (d) low level of oral hygiene indicated by a PBI score above 20 (n = 32); (e) tooth that does not require cuspal coverage; and (f) implant an antagonist (n = 15). Epi Info TM version 7.2.4 (CDC. Atlanta, GA, USA, 2020) was used to compute the required sample size for this study. Following the assumptions of power test 80%, confidence level 95%, exposed-to-unexposed ratio 6:1, and outcome probability of exposed 79% and unexposed 55% based on previous literature, the required sample was 256 [60,61].

Outcome Measures
The primary outcomes of this study were prosthesis survival and success rate. At the annual checkups, the onlays were examined visually with mirror and probe, then the interdental space was examined with dental floss. Each restoration was examined for cracks, fracture, debonding, and marginal color changes. The patients were questioned for postoperative complaints. It was examined if there was need for restoration replacement. Bravo score and Charlie score were recorded.

Data Sources
Two independent investigators (A.L. and M.B.) extracted the patients' data from the hospital database in December 2019. The electronic verification of patients' cards was carried out. The year of onlay delivery was identified, then the annual follow-ups and other visits of the patient were checked. During the annual follow-up sessions, the indirect restorations were examined using the modified United States Public Health Service (USPHS) criteria for retention, color matching, marginal discoloration, marginal adaptation, secondary caries, surface texture, anatomic form, and postoperative sensitivity [62].
Annual checkups had been performed by prosthodontists. Bite-wings or periapical radiographs were taken according to indication of a restorative dentist. Radiographs were then evaluated during retrospective research for the presence of secondary caries by the investigators. This was combined with clinical examination and a decision about the presence of secondary caries was made.

Bias
To reduce the measurement bias, the investigators who checked the protocols in patient cards, were independent and did not work at the University Hospital (A.L. and M.B.); then they discussed the records with the senior investigators (J.S. and B.A.).

Analysis
All statistical tests were executed using the Statistical Package for the Social Sciences (SPSS) version 28 (SPSS Inc. Chicago, IL, USA, 2021) [63]. Primarily, descriptive statistics were carried out to describe the participant demographics and clinical characteristics, as well as their treatment outcomes, using frequencies (n), percentages (%), mean, and standard deviations (µ ± SD). Subsequently, regression analysis was performed for the significant risk factors of restorations failure, and time-to-event (Kaplan-Meier) analysis was executed to compare the restorations survival across age groups, cement materials, and cement viscosity. All inferential tests were conducted with the assumptions of confidence level 95% and significance level < 0.05.

Failure Risk Factors
On running binary logistic regression, gender, arch, and restoration type were adjusted to evaluate the impact of potential risk factors on the odds of restoration failure. The patients older than 55 years had 1.69 (CI 95%: 0.97-2.94) times of adjusted odds ratio (AOR) for restoration failure compared to their younger peers. The resin-based cements had an AOR of 2.90 (CI 95%: 1. 59-5.29) and the cements with low viscosity had an AOR of 2.57 (CI 95%: 1.50-4.41) ( Table 4).

Restoration Survival
A recent meta-analysis of Fan et al. 2021 revealed that survival rate of indirect composite inlays, onlays, and overlays after five years was 91% and the success rate was 84% [64]. Secondary caries and endodontic complications were predominant among composite onlays, and nonvital teeth and multiple-surface restoration were depicted as risk factors [64]. No direct connection was found between bruxism and fractures and no other factors influencing clinical outcome were found in this meta-analysis [64]. Another recent systematic review of Bustamante-Hernandez et al. 2020 concluded that ceramic onlays outperformed composite onlays based on 18 clinical trials, even though ceramic onlays were more prone to fracture and discoloration [65] Survival and success rates of composite onlays were reported by several observational and experimental studies, e.g., Signore et al. 2007 found that indicated composite onlays, after 4-6 years of installation, exhibited a 93% survival rate, with a minority of patients requiring endodontic treatments [66]. Chrepa et al. 2014 evaluated 189 composite onlays received by 153 patients for 24 to 52 months, and they found that 2.1% of the onlays lost retention during the first week, and 1.1% broke up after 26-36 months [67]. While dual-cure self-etching resin cement TotalCem was used in this study, no data were found about gender or age associations with the clinical outcomes of these patients [67].
A randomized controlled trial (RCT) by Fennis et al. 2014 compared the direct versus the indirect composite restorations that provided coverage of the cusp for 5 years [68]. The investigators concluded that the differences in survival rates were not statistically significant in the premolar area where all the restorations were installed, and failure was attributed primarily to the adhesive [68]. Indirect restorations exhibited an 83.2% survival rate for both reparable and irreparable failures, and dislodgement was reported in 26.7% of the indirect restorations, and dislodgement and cohesive failure were reported in 20% [68]. It is worthy of note that dual-cured composite resin Panavia F was used in this RCT [68].
Dias et al. 2018 followed 150 endodontically treated molars and premolars, which received composite overlays [69]. The patients were recalled after two to five years; three reparable fractures (two males and one female) and two irreparable fractures (one male and one female) occurred during the follow-up [69]. There were no fractures of restored teeth nor debonding reported in this study, and all irreparable fractures occurred when the antagonist was an implant-supported ceramic crown [69]. It is worthy of mention that the adhesive cement Relyx Unicem was used in this study [69]. Kaytan et al. 2005 checked every 6 months for two years 94 ceramic onlays received by 47 patients [70]. No debonding, fracture, or discoloration was found in this study, which used dual-cured composite cement [70].
D'Arcangelo et al. 2014 observed 79 indirect composite restorations during five years, and found a 91% survival rate and 84.8% success rate after using preheated light-cured composite [71]. Two patients showed negative pulp vitality tests and four restorations were complicated by secondary caries [71]. One extensive restoration fracture was reported and two restorations lost adhesion between 36 and 48 months of service [71].
In our study, 84.9% of the onlays survived without the need for replacement or repair, and the highest percentage of failure came from 2014 and decreased over time. The restoration fracture caused 50% of our failure cases. These results are consistent with what had been reported earlier by Schulte et al. 2005 andZimmer et al. 2008 [72,73]. However, Zimmer's study was mainly on ceramic onlays and found a high success rate (84.9%) among ceramic onlays after 10 years of follow-up, the study also followed up to 95 patients with 388 composite onlays [73]. The most frequent complication was restoration loss (n = 10), followed by secondary caries that occurred exclusively during the first five years (n = 7), restoration fracture (n = 4), and tooth fracture (n = 2) [73]. Failure rate was found to be higher in the molar area compared to the premolar area, and the Cox regression analysis revealed no association between enamel cervical margin, bruxism, and prior root canal treatment. Moreover, it is worthy of note that only the Vita Cerec Duo cement was used in this study, which is a composite cement [73].
In our study, the higher survival rate was found for onlays compared with overlays; this can be explained in accordance with the systematic review and meta-analysis from Fan et al. 2021, which found that multiple surface restorations tend to have an increased risk of failure [64]. Malament et al. 2021 studied onlay survival for 10.9 years and did not find statistically significant difference depending on age. However, they found the lowest risk of failure for the group under 33 years of age [74]. The suggested hypothesis implies that, in younger age, the loss of hard dental tissue is limited compared to the older age, which can lead to indication of multiple surface restorations with increased risk of failure. This is just a possible explanation and our research cannot support this assumption.

Cement Viscosity
In the in vitro study of Hahn et al. 2001 for the impact of cement viscosity on microleakage, highly viscous cements had significantly better results on the cement/dentin interface [75]. Bortolotto et al. 2013 compared the behavior of composite resin versus resin cement in terms of shrinkage development and early solubility [76]. The lowest shrinkage was observed in composite resin cements, while the shrinkage development was slower in self-cure resin cements [76]. A recent in vitro study of Zeller et al. 2021 studied the viscosity and polymerization of three different composite resin-based cements; the investigators found that there were different reactions for each material upon preheating [77]. For Relyx Ultimate and Relyx Unicem, viscosity rises with preheating to 37 • C; however, the polymerization took place rapidly. These two cements, one adhesive (Relyx Ultimate) and another one self-adhesive (Relyx Unicem), were used in our study [77]. Mounajjed et al. 2018 found in an in vitro study that the marginal gap increased with preheated composite, thus suggesting that more precise placement is possible with highly viscous materials, as achieving smaller gaps can be impossible with preheated composite [42].
In contrast to our findings, Francescantonio et al. 2013 suggested that the smaller layer of cement and low-viscosity cements typically correlate with low polymerization stress and can effectively reduce cracks and premature edge penetrations [78]. Filtek Ultimate Flow, which was a material with low viscosity, failed multiple times in our study. Recently, Marcondes et al. 2020 compared the viscosity of flowable composite resin and resin-based luting cements, and found smaller viscosity in flowable composite [79]. Both groups of materials did not react in the same way upon preheating, as the viscosity of flowable composite was not lower after preheating. This fact for clinicians doubts the philosophy of preheating of composites, unless they do not require the material in detail [79].
Applying ultrasonic vibration by fixation of indirect composite restorations can be an alternative to using preheated composite. Cantoro et al. 2021 found, under electron microscopy, increased homogenous structure and reduced porosity in composite cements as a result of ultrasonic vibration. Nevertheless, this technique was not applied in our study [80]. Sato et al. 2014 suggested that rheology of the luting cements can be modified by adding 10-methacryloyloxydecyl dihydrogen phosphate (MDP), and, when high amounts of residual monomer remain in the cement, the crosslinking of the polymer web can be affected and the long-term stability can be jeopardized [81]. Given the complication of microleakage, the materials with higher viscosity can reduce it [75]. Recently, Zhang et al. 2021 found similar rheological proprieties in flowable resin composites and resin cements [82].

Polymerization
The abovementioned study of Francescantonio et al. 2013 found a higher degree of conversion for higher viscosity of cements activated with light [78]. The polymerization stress is significantly reduced in self-cure cements compared to the light-cured ones. More-over, larger polymerization stress or inadequate polymerization in deeper cavities or under opaque restoration can lead to debonding [83].
Light curing and control above the setting is extremely important for clinicians; because of this reason, thiourethane additives can be used with resin cements. Thiourethane additives can reduce the polymerization stress and increase workability of dual-cured cements and the conversion rate can be increased. However, thiourethane additives affect optical proprieties of the material and reduce transparency [84][85][86][87].

Limitations
The first limitation of this study is related to the prosthetic restorations that were placed by different prosthodontists; therefore, the variability in survival and complications of the evaluated restorations could be partly attributed to the clinical/technical skills of the operators. The second limitation is due to the retrospective nature, which did not allow the investigators to assess subclinical characteristics of the participants.

Conclusions
Indirect composite restoration offers a predictable and minimally invasive solution to hard dental tissue loss. The complications, survival, and success rates can be dependent on multiple factors, including the dentist, patient-related factors, material selection, and other factors. Within the limitations of our study, the composite indirect works present higher longevity when luted with high-viscosity cements. Moreover, the higher longevity was observed when the less invasive solution, e.g., onlays, was used. Based on our study, highviscosity composite-based cements can be recommended for fixation of indirect composite restorations. During the indication of indirect work, it can be recommended to avoid cusp capping when it is not necessary. It can be recommended to pay attention to moisture control in the lower jaw, which is one possible reason for higher failure in the lower jaw.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/ma15010312/s1, Table S1: STROBE Checklist of items that should be included in reports of cohort studies.docx.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.